Production of isoprene



United States Patent 3,327,000 PRODUCTION OF ISOPRENE Willis C. Keith, Lansing, and Byron W. Turnquest, Chicago, Ill., assignors to Sinclair Research, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Mar. 11, 1965, Ser. No. 439,083 12 Claims. (Cl. 260-680) This invention is a continuation-in-part of now abandoned application S.N. 95,523, filed Mar. 14, 1961.

In view of the similarity of cis-polyisoprene, i.e. synthetic natural rubber, to natural rubber, the demand for isoprene is increasing. The availability and cost of isoprene, however, present formidable barriers to the commercial production of synthetic natural rubber. Thermal cracking of olefins such as 2-methylpentene-2 has been reported as a method for isoprene production. The present invention represents an improvement of this method of isoprene production.

The benefit arising from the increased yield of isoprene is obvious. The increased concentration of isoprene in the C product fraction, for example, resulting from the thermal cracking of 2-methyl-pentene-2, may be even more important. When the concentration of isoprene in this C fraction is low, the purification of the former requires the use of expensive extractive distillation techniques. A high concentration of isoprene in the C fraction of cracked products may permit the use of relatively inexpensive standard distillation techniques for purification.

We have now found that the thermal cracking or pyrolysis of an allylic methyl-substituted C aliphatic tertiary monoolefin having an alkyl side chain of 1 to 2 carbon atoms in the presence of an accessory.monoalkene'of 2 to 3 carbon atoms, or a phenyl substituted monoalkene having a total of 8 to 12 carbon atoms, in addition to producing a high, selective yield of isoprene,

results in a high concentration of isoprene in the C product fraction. The alkene portion of the phenyl substituted olefin is preferably of 2 to 3 carbon atoms. The feed of the present invention which is thermally cracked contains a CH radical attached to an allylic carbon atom. An allylic carbon atom is the carbon atom attached to a carbon atom which in turn is attached to the double bond. It is the cracking off of this allylic CH group, i.e. the CH group attached to the allylic carbon atom, that gives isoprene according to this invention. Examples of the allylic monoalkene feeds of the present invention include Z-methyl-pentene-Z; 2,3-dimethyl-butene-1; 2- ethyl-butene-l, 3-methyl-pentene-2 (cis or trans) or mixtures thereof. Although hydrocarbon feedstocks are preferred they might also contain non-interfering substituents.

In accordance with the present invention the allylic monoolefin feed is subjected to thermal'cracking, generally at a temperature of about 700 to 900 C., preferably about 750 to 850 C. in the presence of the accessory olefin and in the vapor phase for a contact time sufiicient to provide high, selective yields of isoprene. Ordinarily a contact time of 0.005 to 2 or more seconds is employed, preferably about 0.01 to 0.05 second. Operating conditions are usually adjusted to give a total hydrocarbon partial pressure of about .005 to 0.5 atmosphere, preferably about .01 to .04 atmosphere. The total pressure is conveniently about atmospheric or slightly above, for instance about 0.1 to 3 atmospheres, preferably about 0.8 to 1.5 atmospheres. There seems to be no reason to govabove about 3 atmospheres, total pressure. The concentration of the accessory olefin in the process of the present invention is governed by the yield increase of isoprene desired with respect to economic factors involved. Generally the accessory olefin will be present in the range of about 1 to 50 moles or more per mole of the allylic olefin; preferably about 3 to 12 moles of accessory olefin per mole of the allylic olefin.

The thermal cracking operation is ordinarily conducted under vacuum or in the presence of an inert gaseous diluent such as steam, and the inert gas will generally be present in a molar ratio of about 1 or 10 to 125 or more moles per mole of allylic monoolefin feed, preferably about 15 to 30 moles per mole of the feed. If desired the inert gas can be employed as the heating medium to bring the feedstock rapidly to the cracking temperatures. This can conveniently be done by heating the inert gas to a temperature above that desired for conducting the cracking operation generally at least C. higher than the reaction temperature and not above say about 200 C. of the reaction temperature. The inert gas is then quickly mixed with the hydrocarbon which is at a temperature below that at which any reactions occur. The temperature to which the gas can be heated can be readily determined from the specific heat of the gas, the molar ratios involved, etc. Similarly, the accessory olefin of the present invention can be used as the heat transfer medium, if desired. Any other heating means known to the art can also be utilized, the means chosen being in some cases a matter of economics. The product from the thermal cracking process can be quenched as with water, for instance, at a temperature below about 260 C. and if desired fractionated in ordinary fractionating equipment to obtain isoprene.

Any source and method known to the art can be utilized to obtain the allylic C tertiary olefin feeds of the present invention. As disclosed in the Turnquest and Verdol application Ser. No. 95,522 filed Mar. 14, 1961, and now abandoned, a suit-able source of the C olefins of the present invention is the mixed hydrocarbon streams boiling primarily in the C range. A petroleum refinery mixed stream boiling in the C range, for instance, usually contains about 5 to 95% tertiary alkenes, preferably 25 to 95%. Other hydrocarbons are generally present in the stream such as non-tertiary olefins, paraflins or other hydrocarbons and their mixtures. Ordinarily these C streams will contain greater than about 10% of 2-methyl-pentene- 2, greater than about 5% of Z-methyl-pentene-l, the percentages of these two olefins generally constituting about 30 to of the total C mixed feed but being no more than about 50 or 60% 2-methyl-pentene-2, about 15 to 50% 3-methyl-pentene-2, about 5 to 35% 2,3-dimethy1 butenes and about 0.5 to 15% 2-ethyl-butene-1. C refinery streams containing the above proportions of tertiary olefins can be derived from gas oil thermal and catalytic cracking operations. A mixture essentially of the allylic olefins of the present invention can be conveniently obtained by causing the tertiary alkenes contained in a mixed hydrocarbon stream boiling primarily in the C range to selectively react with an alcohol of up to about 6 carbon atoms to produce a tertiary C ether product and then decomposing the ether product by contact with acid catalyst at elevated temperatures. The allylic olefins thus formed or separated from mixed hydrocarbon streams by other methods may then be thermally cracked in accordance with the process described above.

The etherification in the above method can be performed, for instance, by using an ion-exchange material in the hydrogen form and in an amount suflicient to catalyze the selective conversion to the tertiary alkyl ether. The ether thus formed can be easily separated from the reaction mixture by distillation and the substantially pure tertiary alkene recovered in good yields by decomposing the ether. The decomposition reaction is carried out by contacting the tertiary ether with an acid catalyst at elevated temperatures to recover the tertiary olefin and alkanol reactants.

The organic hydrogen ion exchange etherification catalysts useful in accordance with the present invention are relatively high molecular weight water-insoluble resins or carbonaceous materials containing an SO H functional group or a plurality of such groups. These catalysts are exemplified by the sulfonated coals (Zeo-Karb H, Naleite X, and Nalcite AX) produced by the treatment of bituminous coals with sulfuric acid, and commercially marketed as zeolitic water softeners or base exchangers. These materials are usually available in a neutralized form, and in this case must be activated to the hydrogen form by treatment with mineral acid, such as hydrochloric acid, and water washed to remove sodium and chloride ions prior to use. Sulfonated resin type catalysts include the reaction products of phenol-formaldehyde resins with sulfuric acid (Amberlite IR-l, Amberlite lR-l00, and Nalcite MX). Also useful are the sulfonated resinous polymers of coumarone-indene with cyclopentadiene, sulfonated polymers of coumarone-indene with furfural, sulfonated polymers of coumarone-indene with cyclopentadiene and furfural and sulfonated polymers of cyclopentadiene with furfural. The preferred cationic exchange resin is a strongly acidic exchange resin consisting essentially of a sulfonated polystyrene resin, for instance a divinylbenzene cross-linked polystyrene matrix having about 0.5 to 20 percent, preferably about 4 to 16%, divinylbenzene therein to which are attached ionizable or functional nuclear sulfonic acid groups. This resin is manufactured and sold commercially under various tradenames, e.g. Dowex 50, Nalcite HCR. This resin, as commercially obtained, has a moisture content of about 50% and it can be used in this form or it can be dried and then used with little or no difference in results ascertainable. The resin can be dried as by heating at a temperature of about 212? F. for 12 to 24 hours or the free water can be removed as by refluxing with benzene or similar solvents and then filtering.

The resin particle size is chosen with a View to the manipulative advantages associated with any particular range of sizes. Although a small size (200400 mesh) is frequently employed in autoclave runs, a mesh size of 20-50 or larger seems more favorable for use in fixed bed or slurry reactors. The catalyst concentration range should be sufiicient to provide the desired catalytic effect, e.g. between about 0.5 and 50 percent (dry basis) by weight of the reactants, with the preferred range being between about 5 to 25 percent (dry basis), for example, percent.

In a continuous reactor the catalyst concentration is better defined by weight hourly space velocity; that is to say, the weight of feed processed per weight of catalyst per hour. A weight hourly space velocity of about 1 to 8 (based on hydrocarbon feed) and up to about 17 based on total hydrocarbon and alcohol feed may be used with advantage. The WHSV can be about 0.1 to 100 based on hydrocarbon feed only, with the preferred WHSV be ing about 2 and 20.

The ether is formed by reacting the tertiary olefin in the hydrocarbon mixture with a primary alcohol, whether monoor =polyfunctional. A ratio of about 0.1 to 100 moles of primary alcohol (or polyol containing primary hydroxyl groups) per mole of tertiary olefin may be used in the etherification with the usual amount being between about 1 and 10 moles of primary alcohol per mole of tertiary olefin, preferably about 5 to 10 moles of the alcohol. A high ratio of alcohol to t-olefin increases the amount of olefin taken from the mixed hydrocarbon feed stream.

Primary alcohols, Whether monoor polyfunctional are effective in the etherification step of this process. Although secondary alcohols do react with tertiary olefins, the conversion rate is too low for practical purposes. Economy and ease of volatilization during the decomposition step generally dictate the use of alcohols of 1 to 6 carbon atoms, and in general, ethanol and methanol are preferred because of enconomy and, usually, they afford higher conversion rates.

The etherification temperature range is about -350 F., with the preferred limit being from about 100225 F. The lower temperature range is preferred, since the formation of the tertiary other is favored, and the formation of dialkyl ether (dimethyl ether in the case of methanol being used as the alcohol reactant) is not significant at lower temperatures. Runs performed at autogenous pressures and others performed under nitrogen pressure of 400-500 p.s.i.g. showed that pressure has no significant effect upon the reaction. The pressure may range from about atmospheric pressure to about 5000 p.s.i.g. or more, with the preferred limits being between about atmospheric pressure and about 600 p.s.i.g. Pressures above atmospheric pressure may be required to maintain the reactants in the liquid phase; however, the reaction can be carried out at autogenous pressure in a continuous system, which is preferred for commercial operation. Batchwi-se reaction in an autoclave is feasible.

The decomposition of the tertiary hexyl alkyl ethers formed may be carried out in autoclave reactor under antogenous pressure, or in a continuous reactor at atmospheric pressure. The tertiary C ethers derived from a primary alcohol and the tertiary olefin undergo the decomposition reaction to form alcohol and mixed tertiary C olefins in the approximate ratios in the mixed hydrocarbon stream. Thus the tertiary ethers have the structure ROR where R is an aliphatic hydrocarbon radical, including cycloaliphatic, of 6 carbon atoms and R is an aliphatic hydrocarbon radical of 1 to 6 carbon atoms.

The temperature range for decomposition of the ethers may vary depending on the particular catalyst employed but generallyfalls in the range of about 100 to 1000" F. Preferred temperatures for a strong acid catalyst system are about 1:00 to 350 F. while the preferred temperatures for a weak acid catalyst are about 450 or 600 F. to 900 F.

The catalyst for the decomposition can be either a strong acid catalyst or a weak acid catalyst that furnishes one or more protons. Included within the strong acid group are organic or inorganic acids such as sulfonic acids, sulfuric acids, phosphoric acids, sodium bisulfite, etc. which have a dissociation constant of at least 1 l0 Useful sulfonic acids include particularly those ion exchange resins described above. The preferred strong acid catalysts are those which are solid at reaction temperatures. Suitable weak acid catalysts are, for instance, solid oxides of a mildly electro-negative element as, for example, boron, silicon, magnesium, aluminum, the elements having the atomic numbers 22 to 28 (Ti, V, Cr, Mn, Fe, Co and Ni) as well as zirconium, molybdenum and tungsten. These catalysts are refractory oxides often of substantial surface area, and many suitable materials are commercially available, some being for use as hydrocarbon cracking catalysts. These cracking catalysts. are frequently composed of mixed acid-acting oxides of metals and semimetals. The major component may be alumina, silica, MgO, B 0 W 0 TiO or V 0 or the acid-acting inorganic solid may be supported on an inert material such as charcoal. The catalyst is usually calcined or activated at a minimum temperature of about 700 F. In any event the catalyst employed in the decomposition will be activated, either before or during initial use, at a temperature of at least that of the decomposition reaction.

A low area cracking catalyst may be used in the decomposition. Such a catalyst will usually have a surface area of less than about 25 square meters per gram, often less than about 1 square meter per gram, and may be a fuse-d silica-alumina material, for instance containing a minor amount of silica, which has been calcined at a temperature of at least about 4000 F. In addition to alumina the fused material may contain minor amounts of other acid-acting solid oxides such as SiO B 0 Cr O TiO and/ or ZrO in a total of about 530%, preferably about -20%. Another low area material which can be employed in the ether decomposition is a-alumina which can be made by calcining alumina hydrates or a-aluminas at temperatures of about 2000 to 3700 F. The family of gamma aluminas can also be used and these are usually 5 obtained by calcination of alumina hydrates at temperatures of about 500 or 700 to 1300 or 1500 F., and have higher surface area and greater activity than the low area catalysts. The high area material frequently has a surface area above about 50400 square meters per gram or even 10 as high as 500' mfi/gm. or more, and gives a higher percentage of dimethyl ether in the decomposition reaction.

Generally, the amount of acid catalyst employed in the decomposition reaction is about 1 to 25 weight percent of the tertiary ether, preferably about 5 to 20%. For a continuous reaction, the amount of catalysts and ether employed usually give a space velocity of about 0.5 to 10 WHSV.

The following examples will serve to better illustrate the present invention.

EXAMPLE I A 12-inch furnace was equipped with a quartz reactor. Ceramic packing was used as a heat transfer medium. 2- methyl-pentene-2 and the accessory olefin were preheated to about 700 F. and then contacted with superheated steam and at about 1470" F. The ratio of steam to dissimilar, i.e. accessory, olefin to feed was 90/ 89.7/ 1. The temperature of the steam was adjusted so that the feed would be at the desired reaction temperature of 1450 F. on entering the quartz reactor maintained at a total pressure of 1 atmosphere. At this temperature and steam-accessory olefin dilution, the feed rate was regulated to give contact times of 0.015 and 0.017 second. The accessory olefin employed is also shown in Table I. After leaving the reaction zone, the gas stream was sampled with a ml. syringe and analyzed. For comparative purposes a run was made without accessory olefin.

The results are shown in Table I.

TAB LE I 6 conditions shown in Table II. The results are also reported in Table II.

TABLE II [Pyrolysis of 2Inethylpentene-2 in the presence of propylene, 1 atm. total pressure, 1,450 F. effect of propylene concentration] Run E F H2O/C =/Cs= 90/4. 2/1 91/20/1 Contact Time, sec-. 0.01 0.014 Conversion of C 50 50 Products, Wt. percent: I

01-0 (2) 3 23.0 (2) 3 23.0 4 31. 3 17.0 38. 7 17.0 Isoprene 43. 1 37.6 45.0 44.0 Other 0 10. 4 9.1 6.1 6.0 C and higher 15. 1 13.3 10. 2 9.9 Moles Isoprene/lOO moles Dimer decomposed. 45.8 54.4 Wt. Percent Isoprene in C Fraction 80.5 88.0

1 Yields based on 2-methylpentene-2 only.

2 Data obtained directly by chromatographic analysis, C1-C3 not; analyzed for.

3 Calculated yields assuming normal C1-C3 yields.

The data of Table II clearly show that the effect of the added accessory olefin is directly related to its concentration, i.e. in small concentration only a small increase in isoprene selectivity is noted (Run E). In large concentration (Run F) a very beneficial effect is found.

EXAMPLE III [Pyrolysis of 2-n1ethylpentene-2 in the prles lesrgel pg additional olefins, 1 atm. total pressure,

Run A B C D Accessory Olefin None 1 Ethylene Propylene a-Methylstyrene H O/Olefin/2-MeC =2 90/ 9.7/1 90/8. 6/1 90/8. 6/1 65/8/1 Contact Time, sec 0. 015 0. 015 0. 015 0. 017 Conversion of C 50 50 50 Products, Wt. percent:

C -C 23.0 23. 0 23. 0 23. 0 C4 17. 0 25.9 17.0 33.1 17.0 31.3 17.0 Isoprene 37 1 43. 8 35. 4 49. 2 44. 3 54.4 47.8 Other C 10 5 11.8 9.6 8.2 7.4 9 1 7.9 C and Higher 12 3 18.4 14 9 9.4 8.3 5 1 4.3 Moles Isoprene/lOO moles Feed Decomposed 45. 8 43 7 54.9 59.0 Wt. Percent Isoprene/in C Fraction. 77. 6 78.6 85. 5 85.6 Isoprene/Z-MeCF 1 16.3 24. 0 18. 6 18. 8 Isoprene/2-C 15. 3 13.2 32. 4 32. 2 Isoprene/2-MeC4=2 ll. 1 13 9 20. 6 20.2

1 Nitrogen used in place of olefin. 2 Yields based on 2-methylpentene-2 only.

4 Calculated yields assuming normal C1-C yields.

The data of Table I clearly demonstrate the effects from the added olefins. Propylene and a-methyl-styrene give marked increase in isoprene yield as well as higher isoprene concentrations in the C range hydrocarbons. The effect of ethylene is much less marked than the other olefins although some increase in concentration of isoprene in the C range is noted.

EXAMPLE II In accordance with the procedure of Example I 2- methylpentene-2 was thermally cracked in the presence of different concentrations of propylene and under the square inch pressure with nitrogen. Analysis of the product by chromatography showed that 36% of the C refinery stream was converted to tertiary hexylmethyl ethers. The ethers were isolated from the reaction mixture by washing out the methanol with water and distilling the residue. The mixed tertiary hexylmethyl ethers were collected at -111 C., n' 1.4013-1.4019.

One portion of the tertiary hexylmethyl ethers obtained from the above extraction was converted to the allylic tertiary C isoolefins by passing the ethers through a reactor comprising a Pyrex tube 240 cm. in length and 2.5 cm. in diameter packed with 50 g. of Dowex 50X-8 catalyst heated in an electric furnace to 200 F. Another portion of the tertiary hexylmethyl ethers was similarly converted but over alpha alumina catalyst at 800 F. The products of the reactions were collected in Dry Ice traps located below the Pyrex reactor tube. The reactor was at atmospheric pressure. The product of each reaction was washed free of methanol with cold water and distilled at atmospheric pressure to give the tertiary C olefins and unreacted tertiary C ethers. Analysis of the olefins formed from each decomposition reaction showed the following:

Tertiary Ca Olofins formed at 200 F. (by decomposition of tertiary Co ethers Tertiary C6 Olefins formed at 800 F. (by decomposition of tertiary C ethers Mixed tertiary C olefins (having approximately the composition of the above C tertiary olefins prepared at 200 F.) were converted to isoprene by passing the C olefins mixed with propylene through a reactor about 15 cm. in length and 1.7 cm. in diameter, packed with ceramic beads. Steam was passed into the reactor system from a preheater section of the reactor at approximately the pyrolysis temperature. Table III below summarizes the results of these runs which were conducted at temperatures ranging from about 1300 to 1390 F. The conversion of the isoolefins to isoprene varied from 30 percent to 45 percent. The contact time in each run about 0.05 second.

TABLE III Run A B C D Temperature, F 1, 300 1, 380 1, 340 1, 390 Pressure, atm 1 1 HzO/C3=/c0 (mole r 73/0/1 85/05/1 75/0/1 79/15/1 Contact Time, sec 0.05 0.043 0. 050 0.043 Conversion, Percent 28 30 43. 6 45 Products, Ultimate Wt., Percent:

C5 c 2. 0 2. 0 Molar Selectivity to Isoprene 48. 0 55. 7 46. 4 61. 4 Percent Isoprene in C Fraction 70. 7 79. 9 70.7 91. 3

O o What is claimed is:

1. In a process for producing isoprene the step consisting essentially of subjecting 2-methylpentene-2 to thermal pyrolysis in the presence of an accessory olefin selected from the group consisting of rnonoalkenes of 2 to 3 carbon atoms and phenyl-substituted monoalkenes having a total of 8 to 12 carbon atoms at a temperature of about 700- 900 C. to produce isoprene, said accessory olefin being present in an amount in the range of about 1 to 50 moles per mole of said 2-methylpentene-2.

2. The process of claim 1 wherein the accessory olefin is a rnonoalkene of 2 to 3 carbon atoms.

3. The process of claim 2 wherein the accessory olefin is propylene.

4. The process of claim 1 wherein the accessory olefin is a phenyl-substituted rnonoalkene having a total of 8 to 12 carbon atoms.

5. The process of claim 4 wherein the phenyl-substituted rnonoalkene is a-methylstyrene.

6. The process of claim 1 wherein the pyrolysis is conducted in the presence of an inert gas.

7. The process of claim 1 wherein the pyrolysis is conducted at a temperature of about 750-850 C.

8. The process of claim 7 wherein the accessory olefin is propylene.

9. The process of claim 1 wherein the amount of accessory olefin is about 3 to 12 moles per mole of said Z-methylpentene-Z.

10. In a process for producing isoprene the step consisting essentially of subjecting a C tertiary rnonoalkene having at least one methyl group attached to an allylic carbon atom, to thermal pyrolysis in the presence of an accessory olefin at a temperature of about 700900 C. to produce isoprene, said accessory olefin being a phenylsubstituted rnonoalkene having a total of 8 to 12 carbon atoms and being present in an amount in the range of about 1 to 50 moles per mole of said tertiary rnonoalkene.

11. The process of claim 10 wherein the phenyl-substituted rnonoalkene is a-methylstyrene.

12. The process of claim 1 wherein the pyrolysis is conducted in the presence of an inert gas at a temperature of about 750850 C.

References Cited UNITED STATES PATENTS 2,404,056 7/1946 Gorin et al. 260-680 2,415,537 2/1947 Schulze et al. 208-128 X 3,104,269 9/1963 Schaffel 260680 FOREIGN PATENTS 588,870 7/1960 Belgium. 831,249 3/ 1960 Great Britain. 868,566 5/1961 Great Britain.

PAUL M. COUGHLAN, JR., Primary Examiner. 

1. IN A PROCESS FOR PRODUCING ISOPRENE THE STEP CONSISTING ESSENTIALLY OF SUBJECTING 2-METHYLPENTENE-2 TO THERMAL PYROLYSIS IN THE PRESENCE OF AN ACCESSORY OLEFIN SELECTED FROM THE GROUP CONSISTING OF MONOALKENES OF 2 TO 3 CARBON ATOMS AND PHENYL-SUBSTITUTED MONOALKENES HAVING A TOTAL OF 8 TO 12 CARBON ATOMS AT A TEMPERATURE OF ABOUT 700900*C. TO PRODUCE ISOPRENE, SAID ACCESSORY OLEFIN BEING PRESENT IN AN AMOUNT IN THE RANGE OF ABOUT 1 TO 50 MOLES PER MOLE AND SAID 2-METHYLPENTENE-2. 